1. Field of the Invention
The present invention relates to communication systems, and, in particular, to mobile telephony systems that conform, for example, to an IS-95 standard.
2. Description of the Related Art
FIG. 1 shows a block diagram of part of a mobile telephony system conforming to an IS-95 standard. In the forward link, signals from base station 108 are transmitted by base station antenna 104 to mobile unit 102. Analogously, in the reverse link, signals from mobile unit 102 are received at base station antenna 104 for base station 108. Antenna 104 is typically hard-wired to base station 108 and signals are transmitted between base station 108 and antenna 104 over that hard-wire connection 106.
The timing of the transmission of forward-link signals within an IS-95 communication system is based on a global positioning system (GPS) time reference (called "GPS 0") derived at the base station, using a GPS receiver (not shown). As described in Section 6.1.5.1 of the IS-95-A Specification, each mobile unit derives its own time reference based on the earliest arriving multi-path component of the forward-link signals transmitted by the base station. Each mobile unit uses its derived time reference to control the timing of the transmission of reverse-link signals.
In an IS-95 communication system, there will be a delay between the time at which forward-link signals are time-stamped by a base station and the time at which the signals are actually transmitted by its antenna. This delay is due, at least in part, to the physical separation between the base station and its antenna, such as that shown in FIG. 1, which separation may vary from base station to base station in a communication system. Processing delay (e.g., from D/A conversion and modulation) also contributes to the duration between time-stamping and actual transmission.
According to Section 7.1.5.2 of the IS-95-A Specification, a base station advances the timing of the forward-link signals by an amount corresponding to the delay between time-stamping and actual transmission, so that the actual transmission of signals by the base station antenna will begin at GPS 0. The magnitude of this timing advancement is based on empirical measurements made when the base station and antenna are first configured. For base stations that have two or more different antennas, the timing advancement is based on the antenna having the smallest delay.
Each mobile unit will derive its own time reference based on the earliest arriving multi-path component of the forward-link signals and use the derived time reference to transmit the reverse-link signals back to the base station. The base station defines a search window within which it scans for reverse-link signals transmitted by the mobile unit. The position of the base station search window is defined with respect to the GPS 0 time reference derived by the base station.
It is desirable to configure each base station with a special test unit, such as test unit 112 of FIG. 1, which is designed for perform remote diagnostic and other maintenance tests on the operations of base station 108 from the central office of the communication system, without interrupting telecommunication service or having to send an operator to the base station. A test unit simulates the operations of a mobile unit to verify that the base station is operating properly.
A test unit may be hard-wired to the base station, as shown by connection 110 in FIG. 1. As such, communication signals between the base station and the test unit will not be transmitted or received by the base station antenna, and there will be minimal signal delay between the base station and the test unit. Like all forward-link signals, the forward-link signals transmitted from the base station to the test unit will be advanced in time to take into account the delay between the base station and its antennas. However, since there is minimal signal delay between the base station and the test unit, the time reference derived by the test unit from the forward-link signals will be earlier than the time reference derived by any of the mobile units by at least the magnitude of the timing advancement applied by the base station. In addition, the reverse-link delay between the test unit and the base station is smaller than the duration from the time that a signal is transmitted from any mobile unit until the time that the signal arrives at the base station.
If the forward-link delay between the base station and the base station antenna is larger than the total round-trip delay between the base station and the test unit (e.g., for configurations in which the antenna is located relatively far away from the base station), the reverse-link signals generated by the test unit may arrive at the base station outside of the base station search window (e.g., before the start of the search window). In that case, the test unit will not be able to establish communications with the base station and the test unit will not be able to perform its diagnostic and other maintenance functions.
For example, if the forward-link delay between the base station and the base station antenna is 100 .mu.sec and the forward-link delay between the base station and the test unit is only 20 .mu.sec, then, advancing the forward-link signals by the 100-.mu.sec forward-link delay, will result in the test unit deriving a time reference equivalent to GPS time -80 .mu.sec. If the reverse-link delay between the test unit and the base station is also 20 .mu.sec, then the reverse-link signals transmitted by the test unit will arrive at the base station at GPS time -60 .mu.sec. Since the base station search window typically starts at GPS time 0, the base station will not be able to process the reverse-link signals transmitted by the test unit.
There are a number of different types of communication systems in which these problems may occur. For example, in a wireless-over-cable communication system, in which communication signals such as PCS (Personal Communication System) signals are transmitted over a conventional cable television plant, signals from mobile units are first received by remote antennas, which send the signals over cables to the base station. In such wireless-over-cable configurations, the base station usually has multiple radiating antenna connectors for the same code division multiple access (CDMA) channel. In addition, the antenna connectors are usually far away from the base station.
Fiber microcell systems differ from wireless-over-cable systems in that fiber microcell systems use only one pair radiating antenna connectors for each sector, and the two antennas are very close together. However, the antennas are typically far away from the base station and fiber is used to connected the pair to the base station.
As such, test units may be unable to initiate and maintain communications with base stations in such wireless-over-cable and fiber microcell systems in which the antennas are far away from the base station.